1. Field of the Invention
Our invention pertains to storage tubes, and in particular to direct-view or display storage tubes adaptable for storage-type oscilloscopes and other instruments. More particularly, our invention concerns a storage target for use in such storage tubes.
2. Description of the Prior Art
According to the typical configuration of a prior art storage target or storage mesh used in a direct-view storage tube for a storage-type oscilloscope, a porous dielectric storage layer is attached to or formed on a fine meal mesh screen or electrode. Usually, the storage mesh is formed by atmospheric vapor deposition of a dielectric comprising magnesium oxide on the metal mesh, as of nickel or copper, while the latter is being supported levelly by a metal frame.
The dielectric is deposited on the metal mesh to a thickness of several tens of microns with a view to minimal electrostatic capacity. Being so porous or loosely packed, the dielectric storage layer of the prior art target has a coefficient of density of not more than 0.1 or so.
By the term "density coefficient", as used herein and in the claims appended hereto, is meant the ratio of the apparent specific gravity of a material or formed product to the intrinsic specific gravity of a closely packed or unintersticed material. Thus any unintersticed or nonporous material or formed product has a density coefficient of one.
Such has been the configuration of the prior art storage target that the dielectric can be deposited to a thickness of only several microns at the maximum in order to form a dielectric storage layer having a higher density coefficient. This is because the dielectric deposited on the nickel or copper mesh to a greater thickness is easy to develop cracks or separate therefrom due to the difference between their coefficient of thermal expansion.
The dielectric storage layer with a sufficiently low density coefficient is practically free from the problems of cracking and separation, even if the layer has a thickness of several tens of microns as above. Because of the low density coefficient, however, the dielectric storage layer provides a low secondary electron emission ratio and low writing speed. The low density storage layer has the additional disadvantage of short service life, being susceptible to "burning" or thermal damage from electron beam bombardment.
A direct-view storage tube of modified design has been suggested in which an additional storage mesh is disposed between and parallel to the first storage mesh and the phosphor screen of the storage tube. The second storage mesh is identical in configuration with the first except that the second has a dielectric storage layer of reduced thickness for an increase in electrostatic capacity. Information is first written on the first storage mesh at high speed. Since the first storage mesh provides a short display persistence, the information is thence transferred on to the second storage mesh for reading.
The direct-view storage tube of this dual storge mesh type also has the disadvantages of low writing speed and the easy burning of the meshes. The use of the two storage meshes, moreover, incurs a substantial decrease in the transmission rate of the electron beams, providing low-output brightness in non-storage operation.
The so-called "reduced mode" of operation has been employed extensively in order to increase writing speed. Let it be assumed that the cathode of the writing gun in a direct-view storage tube has a potential of -2 kilovolts in the normal mode of operation. For the reduced mode of operation a potential of, say, -4 kilovolts is applied to the writing gun cathode, while the deflection system of the storage tube is maintained just as in the normal mode of operation. Through the consequent increase in electron beam density, the writing speed is increased, as from 30 megahertz in the normal mode up to 200 megahertz in the reduced mode.
An objection to this reduced mode of operation is that the storage mesh becomes even more susceptible to burning because of the increased electron beam acceleration energy and increased electron density. Another objection is the inevitable decrease in display area.